The lithium nickel oxide is LiNiO.sub.2 when the ratio of Li (lithium) and Ni (nickel) is 1:1 (atomic ratio) exactly according to the stoichiometric composition ratio, and possesses a lamellar structure same as LiCoO.sub.2 or the like, and hence its utility as cathode active material for lithium secondary battery is expected.
However, in the conventional synthesis of lithium nickel oxide, the ratio of Li and Ni tends to be deviated from the stoichiometric composition ratio, and Ni invades into the Li layer and the lamellar structure is disturbed, and therefore when used as a cathode active material for a lithium secondary battery, the charging and discharging capacity is smaller than the value expected from LiNiO.sub.2 with Li and Ni atomic ratio of 1:1.
Specifically describing the method of synthesis of conventional lithium nickel oxide and the Li/Ni atomic ratio of thus synthesized lithium nickel oxide, hitherto, the lithium nickel oxide was synthesized by heating the lithium hydroxide hydrate (LiOH.H.sub.2 O) and nickel (Ni) powder in oxygen (O.sub.2) atmosphere at 750.degree. C. for 12 hours, pulverizing again, and further baking, as disclosed, for example, in Japanese Patent Tokkosho No. 63-59507.
What is actually obtained is, however, not LiNiO.sub.2 at the stoichiometric composition ratio of 1:1 (atomic ratio) of Li and Ni, but instead a composition of Li.sub.0.85 Ni.sub.1.15 O.sub.2, having a Li/Ni ratio (atomic ratio) is 0.74, in which the lamellar structure was disturbed, and when used as cathode active material for lithium secondary battery, only a battery of small charging and discharging capacity was obtained as mentioned above.
If Li/Ni ratio (atomic ratio) is 1 according to the stoichiometric composition ratio, since it is possible that the crystal configuration of Li and Ni is random or disorder and it is believed that a part of Ni may invade into the layer of Li, thereby charging and discharging capacity being decreasing.
It is hence the first object of the present invention to provide a lithium secondary battery having a large charging and discharging capacity by solving the problem of small charging and discharging capacity of the lithium secondary battery when the lithium nickel oxide synthesized in the conventional method is used as cathode active material.
It is also the second object of the present invention to provide a cathode active material used in said lithium secondary battery.
Moreover, it is the third object of the present invention to provide a method for manufacturing the cathode active material used in said lithium secondary battery.